Network ring topologies are gaining in popularity, particularly in Internet Protocol (IP) networks. Such networks enable carriers to offer large bandwidth to users in a cost-effective manner. In order to gain these benefits, however, IP needs appropriate support at the Media Access Control (MAC—protocol layer 2) level, to provide functions such as load balancing, protection and clock synchronization.
One solution that has been proposed to meet these needs is the Spatial Reuse Protocol (SRP), which is described by Tsiang et al., in Request for Comments (RFC) 2892 of the Internet Engineering Task Force (IETF). This document, which is available at www.ietf.org/rfc.html, is incorporated herein by reference. SRP relates to the ring network as two overlapping local area networks (LANs), identified arbitrarily as an inner ring and an outer ring. In one of the rings, communication flows clockwise, while in the other it flows counterclockwise. Each node in the ring can communicate directly with all other nodes through either of the rings, using the appropriate MAC addresses of the nodes. Spatial reuse enables different nodes to use different, non-overlapping spans of the same ring simultaneously (unlike earlier ring protocols), thus increasing the overall aggregate bandwidth that is available.
Although SRP allows nodes to choose whether to route their packets on the inner or the outer ring, it does not provide any method for nodes to use in deciding which ring to choose. SRP recognizes different packet priority levels, and it also provides a mechanism for congestion control and fair load control on each of the two rings individually at the lower priority level. It thus supplies an infrastructure that can be used by applications to offer different Quality of Service (QoS) options. The SRP RFC, however, does not deal with how IP flows should be assigned to high priority levels or how load control should be accomplished for such high-priority packets.
There are routing protocols known in the art for choosing an optimal path between two nodes when multiple paths are available. These protocols have generally been designed with mesh networks in mind, although they can also be applied to bi-directional ring networks. For example, the Open Shortest First Protocol (OSFP) is a link-state routing protocol that is used to identify and select the path that has the lowest overall “cost.” OSFP is described by Moy in RFC 2328 of the IETF, which is available at the above-mentioned Web address and is likewise incorporated herein by reference. This protocol enables a system administrator to assign a cost to each link in the network, so that low-cost links are the ones most likely to be selected for routing. OSFP does not specify, however, how the costs are to be determined. Moreover, since the costs are assigned statically by the system administrator, the protocol does not provide any means or basis for updating the costs automatically, while the network is running, in response to network resource constraints.